A magnetic sensor is an electronic device for converting a detected amount such as electromagnetic force (for example, current, voltage, electric power, magnetic field or magnetic flux), dynamic quantity (for example, position, speed, acceleration, displacement, distance, tension, pressure, torque, temperature or humidity) or biochemical quantity, into voltage through a magnetic field. The magnetic sensors are classified into a Hall sensor, an anisotropic magnetoresistive sensor (herein after may also be referred as AMR sensor), a giant magnetoresistive sensor (herein after may also be referred as GMR sensor) and the like, depending on a method for detecting the magnetic field.
Among these sensors, GMR sensors are advantageous in that:
(1) GMR sensors have an extremely large maximum value in the rate of change in electrical resistivity (that is to say, (MR ratio=Δρ/ρ0 (Δρ=ρH−ρ0, wherein ρH is electrical resistivity where an external magnetic field is H, and ρ0 is electrical resistivity where an external magnetic field is 0) in comparison with AMR sensors;
(2) GMR sensors have a small temperature change in resistance value in comparison with the Hall sensor; and
(3) GMR sensors are suitable for miniaturization, because the materials having a giant magnetoresistive effect (herein after may also be referred as GMR effect) are a thin-film material. Accordingly, GMR sensors have been expected to be applied as a high-sensitivity micromagnetic sensor which is used in computers, electric power equipment, automobiles, home domestic equipment, portable equipment and the like.
The materials known to show the GMR effect include a metal artificial lattice composed of a multilayer film having a ferromagnetic layer (for example, a permalloy layer) and a non-magnetic layer (for example, a Cu, Ag or Au layer) or a multilayer film with a four layer structure (so-called “spin valve”) having an antiferromagnetic layer, a ferromagnetic layer (a fixed layer), a non-magnetic layer and a ferromagnetic layer (a free layer); a metal-metal-based nano-granular material including nanometer-sized fine particles composed of a ferromagnetic metal (for example, permalloy) and a grain boundary phase composed of a non-magnetic metal (for example, Cu, Ag or Au); a tunnel junction film causing a MR (magnetoresistive) effect by a spin-dependent tunneling effect; and a metal-insulator-based nano-granular material including nanometer-sized fine particles composed of a ferromagnetic metal alloy and a grain boundary phase composed of a non-magnetic insulating material.
Among these materials, multilayer films represented by the spin valve are generally have a feature that they are high in sensitivity in a low magnetic field. However, multilayer films are poor in stability and yield, and have a limit for restricting manufacturing cost, because it is necessary to laminate thin films composed of various materials with a high degree of accuracy. Accordingly, multilayer films of this kind are exclusively used only for high-value added devices (for example, a magnetic head for a hard disk), and are considered difficult to be applied to magnetic sensors which are forced to make competition in price with AMR sensors or hall sensors having a low unit price. Further, since diffusion tends to occur between layers of the multilayer film and the GMR effect tends to disappear, the multilayer films have a significant drawback of poor heat resistance.
On the other hand, nano-granular materials are generally easily manufactured and have good reproducibility. Accordingly, when the nano-granular materials are applied to the magnetic sensors, the cost of the magnetic sensors can be decreased. In particular, the metal-insulator-based nano-granular materials are advantageous in that:
(1) the metal-insulator-based nano-granular materials show a high MR ratio exceeding 10% at room temperature, when a composition thereof is optimized;
(2) the metal-insulator-based nano-granular materials have an outstandingly high electrical resistivity ρ, so that microminiaturization and low power consumption of the magnetic sensor are realizable at the same time; and
(3) the metal-insulator based nano-granular materials can be used even under a high temperature circumstance unlike the spin valve film containing an antiferromagnetic film which is poor in heat resistance. However, the metal-insulator-based nano-granular materials have a problem that magnetic field sensitivity is extremely small in a low magnetic field. Accordingly, in such a case, yokes composed of a soft magnetic material are disposed on both ends of a giant magnetoresistive film (herein after may also be referred as GMR film) to increase the magnetic field sensitivity of the GMR film.
Various proposals have hitherto been made for a thin-film magnetic sensor in which the yokes composed of the soft magnetic material are disposed on both ends of the GMR film and a method for manufacturing the same.
For example, JP-A-2004-363157 discloses a method for manufacturing a thin-film magnetic sensor including: (1) forming a protrusion on a surface of a substrate, (2) forming thin-film yokes on both ends of the protrusion, and (3) forming a GMR film on a leading end surface of the protrusion and surfaces of the thin-film yokes adjacent thereto. This document describes that (a) the GMR film having a uniform thickness can be formed over the length of a gap and that (b) electric and magnetic characteristics of the thin-film magnetic sensor is stabilized, owing to such a method.
Further, JP-A-2006-351563 discloses a thin-film magnetic sensor in which a barrier layer is formed between a GMR film and a substrate.
This document describes that when the barrier layer is provided between the GMR film and the substrate, annealing-derived changes in the rate of change in electric resistance R and the rate of change in magnetic resistance of the GMR film after annealing become approximately equivalent to those in the case of the GMR film alone.
Furthermore, JP-A-2003-78187 discloses a thin-film magnetic sensor in which soft magnetic thin films are formed on both ends of a GMR film, and hard magnetic thin films are further formed on undersurfaces of the soft magnetic thin films.
This document describes that when a bias magnetic field is applied to the soft magnetic thin films by using the hard magnetic thin films, the magnitude of an external magnetic field and polarity can be detected at the same time